1. Field of the Invention
The invention relates to a cold cathode, and more particularly to a field emission cold cathode acting as an electron emitter.
2. Description of the Related Art
A cold cathode array comprising a plurality of fine cold cathodes arranged in an array has already been suggested by C. A. Spindt in "A Thin-Film Field-Emission Cathode", Journal of Applied Physics, Vol. 39, No. 7, pp. 3504-3505, 1968. Each of the fine cold cathodes has a fine conical emitter, and a gate electrode located in the vicinity of an associated cold cathode and having a function of generating a current through an emitter and controlling the thus generated current. This type of cold cathode array is called Spindt type cold cathode, and provides advantages that a higher current density can be obtained than that of a hot cathode, and that a velocity distribution of emitted electrons is relatively small.
In addition, the above-mentioned cold cathode array has smaller current noises than that of a conventional field emission type cold cathode used in an electron microscope, and can operate at a small voltage, for instance, in the range of a couple of volts to 200 volts. Furthermore, whereas a single field emission type cold cathode used in an electron microscope requires a ultra-high vacuum of about 10.sup.-8 Pa for operation thereof, the above-mentioned cold cathode array can operate in a sealed glass tube having a vacuum of about 10.sup.- 4 Pa to about 10.sup.-6 Pa by having a gate electrode situated in the close vicinity of an emitter and further by having a plurality of emitters arranged therein.
FIG. 1 is a cross-sectional view illustrating a part of a conventional Spindt type cold cathode array. The illustrated cold cathode array includes a silicon substrate 101, a plurality of fine conical emitters 102 formed on the silicon substrate 101 by vacuum evaporation, and having a height of about 1 .mu.m, an insulating layer 104 formed on the silicon substrate 101 around each one of the emitters 102, and a gate electrode 103 formed on the insulating layer 104. A plurality of cavities 105 are formed throughout the gate electrode 103 and the insulating layer 104 so that a surface of the silicon substrate 101 is exposed. Each one of the emitters 102 is formed on an exposed area of the silicon substrate 101 in each one of the cavities 105.
The silicon substrate 101 and the emitters 102 are in electrical communication with each other. Specifically, a dc voltage of about 100 V is applied between (a) the silicon substrate 101 and the emitters 102 and (b) the gate electrode 103 in such a manner that the gate electrode 103 is positive. The silicon substrate 101 is spaced away from the gate electrode 103 by a distance of about 1 .mu.m, and each one of the cavities 105 is designed to have a diameter of about 1 .mu.m. In addition, each of the emitters 102 is designed to have a quite sharpened apex. Hence, an intensive electric field is applied to the apexes of the emitters 102. When an electric field applied to the apexes of the emitters 102 has an intensity in the range of 2.times.107 V/cm to 5.times.107 V/cm, electrons are emitted from the apexes of the emitters 102. As a result, there can be obtained a current in the range of 0.1 .mu.A to a couple of tens of micro-amperes per an emitter.
By arranging a plurality of fine cold cathodes having the abovementioned structure, on the silicon substrate 101 in an array, there is constituted a planar cathode generating a large amount of current. The Spindt type cold cathode as mentioned above may be applied to an electron tube such as a planar display, a fine vacuum tube, a micro-wave tube and a cathode ray tube, or to an electron source to be used for a variety of sensors.
A field emission type cold cathode is generally designed to have a structure wherein an emitter is spaced away from a gate electrode by a distance of a micrometer order to a sub-micrometer order, and an emitter has a sharpened apex, to thereby cause an intensive electric field to apply to an apex of an emitter. Accordingly, a discharge is difficult to occur between an emitter and a gate electrode in poor vacuum in operation. If a discharge in poor vacuum continued, an emitter and hence a gate electrode and an insulating layer located around the emitter would be molten, resulting in that an emitter and a gate electrode are short-circuited therebetween.
U.S. Pat. No. 4,940,916 to Borel et al. has suggested a solution for preventing a short-circuit between an emitter and a gate electrode, caused by continued discharge. In the suggested solution, a resistive layer is formed on a substrate just below an emitter, and an electrically conductive pattern for providing a current to an emitter is in the form of a mesh.
However, this solution is accompanied with a problem that the conductive pattern mesh makes it impossible to enhance a cold cathode arrangement density. In addition, an emitter arranged in the center of the conductive pattern mesh would have a higher resistance than a resistance of emitters arranged in a marginal area of the conductive pattern mesh, and as a result, would be quite difficult to emit electrons therefrom.
The inventors have suggested a field emission type cold cathode device in Japanese Patent Application No. 8-133959 in order to solve the abovementioned problems. FIG. 2 illustrates the suggested field emission type cold cathode device, which includes a silicon substrate 101, a plurality of fine conical emitters 102 formed on the silicon substrate 101, an insulating layer 104 formed on the silicon substrate 101 around each one of the emitters 102, and a gate electrode 103 formed on the insulating layer 104. A plurality of cavities 105 are formed throughout the gate electrode 103 and the insulating layer 104 so that a surface of the silicon substrate 101 is exposed. Each one of the emitters 102 is formed on an exposed area of the silicon substrate 101 in each one of the cavities 105. The suggested cold cathode device further includes an insulating layer 106 filled in a trench formed in the insulating layer 104 and the silicon substrate 101 so that the trench surrounds each one of the cavities 105.
In the illustrated cold cathode device, since a region immediately below the emitter 102 is surrounded by the insulating layer 106, carriers do not spread towards a surface of the silicon substrate 101, and accordingly, it is possible to avoid a resistivity of the silicon substrate 101 from being decreased. Thus, even if a discharge was made to occur, it would be possible to keep a resistivity of the silicon substrate 101 substantially constant. As a result, a peak current in a discharge can be suppressed.
In addition, since the resistivity of the silicon substrate 101 is divided into pieces by the insulating layer 106 surrounding the cavities 105 therewith, a voltage drop caused by the divided resistivity in normal operation of the cold cathode is smaller than a voltage drop occurring in the above-mentioned resistive layer in U.S. Pat. No. 4,940,916. Specifically, the former is one-Nth of the latter wherein N is the number by which the resistivity is divided. Furthermore, it is not necessary for the cold cathode device illustrated in FIG. 2 to have a horizontal length to form the resistive layer therein, and hence the cold cathode device can enhance a device arrangement density.
In the field emission type cold cathode device illustrated in FIG. 2, since the silicon substrate 101 is divided into blocks each including the emitter 102, it is possible to cause the voltage drop in each of the blocks to be small. However, when electrons are emitted from each of the emitters 102 in normal operation of the cold cathode device, depletion regions 107 are generated on inner surfaces of the insulating layer 106, as illustrated in FIG. 3. As a result, each one of the blocks has a greater resistivity.
The depletion regions 107 are generated because a voltage difference between a first block in which the emitter 102 is formed and a second block which is located adjacent to the first block through the insulating layer 106 and in which no emitters 102 are formed. Specifically, when electrons are emitted from the emitters 102, there occurs a voltage drop due to a resistance in the blocks located outermost. As a result, a voltage just below the emitters 102 emitting electrons therefrom is dropped, which causes a voltage difference between a block in which the emitter 102 is formed and the silicon substrate 101 which is located adjacent to the block through the insulating layer 106, but in which no emitter 102 is formed. The thus generated voltage difference causes the depletion regions 107.
As a larger amount of electrons is emitted from the emitters 102, thicker depletion regions 107 are formed, and finally, an emission current is saturated, as illustrated in FIG. 4. As a result, there is produced uniformity in an emission current between first block located outermost and second blocks located inside the first blocks among the blocks formed by dividing the silicon substrate 101.
If a cold cathode having the above-mentioned uniformity in an emission current between the first and second blocks was applied to a display such as a planar display, there would be caused uniformity in brightness of images in a display area, which would significantly deteriorate image quality.